1. Field of the invention
The invention concerns an electronic control-circuit for the supply of ohmic-inductive loads by means of direct-current pulses with variable pulse duty-ratio--in particular of electronic motors, preferably those in whose stator or rotor a magnetic unidirectional field is produced; with a regulator that features a transformer; wherein on the primary side the transformer is connected in push-pull arrangement--via electronic switches that are bridged by freewheeling diodes--to a source of d.c. voltage; and on the secondary side is in connection--via electrical switches that are symmetrically connected to the transformer and[are bridged by freewheeling diodes--to the load; and wherein the transformer is preferably designed as an autotransformer with single continuous winding with center tap and taps that are symmetrical to the latter.
2. Description of the Prior Art
So-called step-down regulators [choppers], which consist of bipolar transistors, MOS field-effect transistors, or thyristors make it possible to achieve economically a partial-load operation of consumers, with supply from a source of d.c. voltage with constant voltage, because in this case one avoids losses such as those that occur in adjustable resistances which are inserted in the circuit ahead of the arrangement. This is of importance in such cases as, for instance, the operation of electric vehicles from batteries carried along, because by means of a low-loss partial-load operation such vehicles can greatly improve the autonomy achievable with one and the same battery charge.
In the most simple case, an electronic switch is in series with the source of d.c. voltage and with the electric motor, with an additional freewheeling-diode connected in parallel with the motor. The electrical energy which is supplied to the motor via the switch can be controlled by means of the pulse duty-ratio, between the closed and open condition of the switch. This is the most simple mode of operation, which corresponds to one-quadrant operation.
If the motor lends itself to being operated as a generator, independently of its speed of rotation, a two-quadrant operation is possible. In this case, the motor is not only electrically driven, but can also be braked electrically. In that case, the control circuit for supplying the motor encompasses a series-circuit of two electronic switches that is connected to the source of d.c. voltage, the electric motor being connected in parallel to one of these two switches. At any given moment, only one of the two electronic switches may be made conductive, while the other one must be blocked. The electronic switch connected in series with the source of d.c. voltage and with the motor is used for driving operation, as described earlier. For braking operations, the switch that is connected in series with the source of d.c. voltage and with the motor is opened, the motor operating as a generator; it may be short-circuited with the electronic switch that is in parallel with it. Assuming that the exciting fields stay the same, the voltage of the generator has the same polarity. However, the flow direction of the current is opposite to the one that occurs during operation as a motor. This means that in the case of-a d.c. series-wound motor, the field winding must have its poles reversed for braking purposes. On the other hand, in the case of a motor with shunt characteristics--such as, for instance, a permanent-field motor--no special measures must be taken. By means of the short-circuiting of the motor which is functioning as a generator, a current that rises with time flows from the generator through the smoothing choke (which may be made up, for instance, by the motor winding); when the current circuit is opened, a self-induced voltage appears at the smoothing choke, which voltage, together with the generator voltage, exceeds the voltage of the source of d.c. voltage. This causes a flow of current which diminishes with time, flowing via the freewheeling diode of the other electronic switch provided for motor operation, towards the source of d.c. voltage (for instance, a battery), thus producing a recovery of energy. This is equivalent to two-quadrant operation. Upon reversing the direction of energy flow, the step-down regulator [chopper] becomes a step-up regulator [chopper]. In this fashion, even with very small generator voltages (low speed of rotation and/or low driving speed), energy can be regenerated.
If the direction of rotation of the motor is to be reversible, the voltage supplied to the motor must be capable of having its polarity inverted. This can be achieved by connecting, to the source of d.c. voltage, two parallel series-circuits with two electronic switches each, in a bridge-circuit fashion, and by connecting the motor as a bridge diagonal. The circuit is constructed symmetrically, whereby one bridge branch is operated, as described earlier, while the other branch provides a permanent connection with the chassis ground. Upon inverting the direction of rotation, the functions of the two branches are exchanged among themselves. This is equivalent to a four-quadrant operation.
In the case of more recent motor designs, it has been found advantageous to avoid the use of wear parts which feature a limited useful life--such as the commutator and, in particular, the brushes--and to replace these by electronic switches. This also improves commutation, with the elimination of any movable parts that carry current to the rotor, if the latter is equipped with permanent magnets and is provided with windings housed in the stator. In that case, one speaks of commutator-free direct-current motors,, or of electronically commutated direct-current motors (although these actually are synchronous motors with permanent excitation). In such a case, a selsyn is provided, with which the control of the windings is synchronized with the rotational position. In connection with an electronic control-circuit for the supply of such motors, the commutating switches may also be simultaneously used as switches for a step-down regulator [chopper]. With an appropriate design of the motor, the inductances of the winding may be used as smoothing chokes. For triggering purposes, an alternating-current bridge circuit should be provided.
If the nominal voltage of the motor is not approximately equivalent to the voltage of the source of d.c. voltage, this situation can be solved by using a so-called d.c. converter with a transformer. In the case of larger capacities, in particular, it is practical to use a push-pull circuit with symmetrical operation, in which circuit, once again, two electronic switches are provided on the secondary side of the transformer, which switches are led together at one point, between which point and the chassis-ground the motor is connected. The electronic switches on the primary side are turned-on and turned-off alternatingly, though not following each other continuously in a gapless manner. In this fashion, the voltage of the source of d.c. voltage is transformed in accordance with the voltage ratio r of the transformer, in each case the freewheeling diode of one of the two electronic switches on the secondary side being conductive, and the current rising through the smoothing choke and the load. Immediately after turning off the respective electronic switch on the primary side, the current can continue to flow through the smoothing choke, in that the current divides up symmetrically over the two winding halves of the transformer and over the two freewheeling diodes of the electronic switches on the secondary side. In this situation the transformer is being operated in a bifilar manner, and there is no induction effect on the primary side. Furthermore, the decaying magnetizing current of the transformer superposes in an asymmetric manner. During braking with the regeneration of energy, the electronic switches on the primary side remain open. In that case, at first the two electronic switches on the secondary side are closed, the current in the smoothing choke increases, and the transformer is therefore being operated in a bifilar manner so that no inductive effect occurs. Immediately thereafter, one of the two electronic switches on the secondary side is opened, whereby the sum made up of the inductive voltage of the smoothing choke and the generator voltage is transformed back into the source of d.c. voltage, and the flow of current into the source of d.c. voltage is made possible, via the freewheeling diode of one of the two electronic switches on the primary side. Such a converter is also appropriate for two-quadrant operation.
In principle, almost any voltage conversion can be carried out with the known circuit arrangements, as long as an appropriate pulse duty-ratio is set for the electronic switches. With the additionally provided transformer of the d.c. converter, a fixed adjustment can be carried out over a wide range. However, the dynamic range (from the lowest to the highest input-voltage) must be carried out via the variation of the pulse duty-ratio. This represents a weak point of the known circuit arrangements: while it is true that extreme pulse duty-ratios can be set (for instance, values below 0.1 or over 0.9), nonetheless the efficiency changes drastically at the boundaries of the range. This is connected with the fact that the power transferred is proportional to the mean value of the current, but the power loss in the electronic switches made up of transistors is proportional to the square of the rms current. The ratio of mean value to rms value deteriorates with an increasing pulse duty-ratio p or (1-p).
The invention aims at providing an electronic control-circuit for the supply of ohmic-inductive loads by means of direct-current pulses with a changeable pulse duty-ratio, which [electronic control-circuit] will permit that as large as possible a dynamic voltage range be achieved, without an excessive variation of the pulse duty-ratio. Such an arrangment is necessary in particular in the case of vehicle drives.